1. Field of the Invention
The present invention relates to sensors and sensing methods. More specifically, the invention relates to methods and apparatus for sensing and monitoring the accumulation of impurities or other film inside a process chamber.
2. Description of the Background
Manufacturers of semiconductor integrated circuits (ICs) are faced with intense competitive pressure to improve their products and as a result, pressure to improve the processes used to fabricate those products. This pressure in turn is driving the manufacturers of the equipment used by IC manufacturers to improve the performance of their equipment, and in particular to reduce the operating cost to users of their equipment.
One particular type of tool that is widely used, and is therefore particularly susceptible to these competitive pressures, is the plasma reactor. These reactors are used to remove material from a wafer by a process called plasma etching. The mechanisms of plasma etching are complex and it is essential to maintain and control the plasma parameters and chamber conditions. Maintenance and monitoring of plasma process chamber conditions is the focus of significant technological development in the industry.
One of the key factors affecting product quality and the productivity of plasma processes is the presence of defect forming particles and related contamination within a chamber within a plasma reactor. The accumulation of polymers and other byproducts of the etch process from process chamber components causes yields to drop and maintenance expenses to increase. The manufacturer of plasma reactors that can address these issues and consistently demonstrate superior process control and product quality is positioned to expand market share.
A first problem is how to monitor the accumulation of a film on plasma tools inside a plasma process chamber. During an etch process, complex chemical processes take place, including the chemical transformation of photoresist on the surface of a wafer being etched, the removal of surface material via mechanical and/or chemical processes. These processes create chemical species that deposit on the walls of the chamber and other surfaces within the chamber. This material accumulates over the course of etching many wafers until it reaches a thickness at which the film under internal stresses breaks up and flakes off. These flakes can then move around the chamber, landing on the production substrate, leading to an immediate defect. These flakes can also land on other vital surfaces such as system probes, where they can adversely affect system performance.
One way to solve the same problem is to dismantle the process chamber and visually inspect the pieces. However, this is extremely inefficient and costly because of the extended chamber downtime. Another way to solve the same problem is to use a spectrometer to look at emission of specific species. However, this can be more complicated and requires adjustment when utilizing different plasma conditions.
Therefore, there is a need in the art to simply and efficiently monitor the growth or accumulation of a film on plasma tools inside a plasma process chamber. Three patents disclose arrangements for monitoring the accumulation of films.
U.S. Pat. No. 6,146,492 (Cho et al), “Plasma process apparatus with in situ monitoring, monitoring method, and in situ residue cleaning method,” appears to describe a plasma process apparatus and in situ monitoring method that is a complex approach having the disadvantage that a reactive gas must be injected into a chamber after a process has been completed, to allow the exiting gas to be analyzed to decipher the thickness of the wall film. A simpler approach allowing measuring of film accumulation is needed.
U.S. Pat. No. 6,025,916 (Quick et al.), “Wall deposition thickness sensor for plasma processing chamber,” appears to describe a device for measuring polymer build-up on plasma chamber walls that involves the complex and indirect approach of monitoring interference patterns of light passing through a chamber window. A simpler and more direct approach is desirable.
U.S. Pat. No. 5,948,983 (Gogol, Jr. et al.), “Wall deposition monitoring system,” appears to describe a wall deposit monitoring system for measuring variation in wall deposit thickness in an etch or deposition chamber. This complex method requires installation of a piezoelectric sensor on the chamber wall, and indirectly monitors contaminants by measuring vibration damping created by film accumulation. Again, a simpler and more direct approach to measuring film accumulation is desirable.
A second problem is how to optimize the scheduling of plasma chamber cleaning. Maintaining a clean chamber in a plasma etch tool is critical to producing integrated circuits (ICs) in a plasma etching process. Production efficiency depends in particular on the cleanliness of the process chamber. Conventional techniques for ensuring a clean chamber include operating the plasma chamber for a predetermined time and then dismantling the chamber for visual inspection. This technique does not account for changing plasma conditions and does not result in an accurate real-time representation of the chamber cleanliness. Inaccurate representations of the chamber cleanliness can result in product defects. Therefore, it is necessary to properly schedule maintenance to maximize production and maintain product quality. Therefore, there is a need in the art to optimize scheduling of plasma chamber cleaning.
A third problem is how to lower the cost of plasma etch processes. Maintenance and other non-productive time in a plasma etch system is extremely costly for manufacturers of semiconductor products. In addition, any defective product wastes valuable time and resources, increasing production costs. Therefore, there is a need in the art to lower the cost of plasma etch processes.